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P.2 DISCUSSION OF MINERAL GROUPS
P.14
P.2.1 Silicate Minerals
Silica, Si0 2, makes up over 60% of the earth's crust, and alumina, A1 20 3 , makes up
another 15%. It is not surprising that these elements dominate the rock-forming minerals.
About half of the known mineral species are alumino-silicates, most of which are composed
of one or more of the other eleven most abundant elements in the earth's crust. Feldspar
alone makes up 58% of the earth's crust. Because of the abundance of these silicate min-
erals and their occurrence in a wide variety of rocks, one naturally asks if any of them
might be potential radionuclide hosts. Detailed chemical and crystallographic data on most
of the silicate minerals have been compiled by Deer, Howie, and Zussman (1962).
The suitability of silicates as hosts depends specifically on the ability of the radio-
nuclide to substitute in solid solution for one of the essential ions of the compound. This
is especially true for the common rock-forming silicates. We examine each of the major
groups of silicate minerals and consider the general principles of crystal chemistry that
might elucidate any ionic substitutions of interest. We also consider some common families
of silicate minerals that may to have potential as repository minerals.
We can dismiss some groups quite easily. The silica (Si0 2 ) family of minerals is
usually rigidly stoichiometric, although substitutions of Al for Si create a charge
+ +
imbalance; this is usually compensated for by "stuffing" the framework with Na , K or
Ca+. Cs+ and Sr+ 2 are too large to enter into these compounds. The olivine-related min-
erals, including the humite series, are structurally based on close packaging of oxygen
ions, and the largest ion that finds its way into these compounds is (Ca+2)VI at 1.00 A.
Only Tc +4 is small enough to fit comfortably, but it is too highly charged. The lanthanide
and actinide elements likewise are too highly charged.
P.2.1.1 Pyroxene Minerals
The pyroxene group of minerals are a series of compounds with a general formula
XY(Si,A1) 0 6, where X represents usually a mono- or di-valent ion with ionic radius in the
2 + +2 +2 +2 +2 +
range 0.6 to 1.0 A. Examples are Na , Ca +2 , Mn , Fe +2 , Mg 2 and Li . The Y cations are
di- or tri-valent ions with radii in the range of 0.5 to 0.8 A. Examples include Mn , Fe ,
Mg 2 , Fe +3 , A1+ 3 , Cr +3 , and Ti +4 . These small ranges in ionic size result from a structure
that is quite closely packed in terms of the oxygen ions. Too much distortion from substitution
of larger ions usually breaks down the structure.
About the only critical element which might substitute in pyroxene would be Tc + 4 with
an ionic radius of 0.6 A. The only other 4-valent ion that occurs in pyroxenes is Ti 4
(radius--O.605 A). Titanium rarely substitues in quantites greater than one percent by
weight, although in some of the titanaugites it may reach 3 to 5%.
The suitability of pyroxene as a technetium host require considerable research and, as
a host, pyroxenes are marginal. It is probable that ferrite-like phases will prove more
suitable hosts for technetium than any silicate.